1. Field of the Invention
This invention relates generally to a rotating phased-phased array radar, and in particular to a method of generating accurate estimates of azimuth and elevation angles of a radar target using pre-calculated monopulse curves for a rotating monopulse radar using coherent integration of pulse returns.
2. Description of Related Art
An antenna of a mechanically rotating radar moves in azimuth relative to a target as pulses are transmitted and received. As a result, the pulse returns are scan-modulated by the radar's two-way antenna patterns. In other words, the pulses experience dissimilar antenna pattern gains as the array rotates. Coherent integration of the pulse returns is done to increase the signal-to-noise ratios (SNRs) of the received signals prior to detection and target angle estimation. Non-rotating radars frequently employ the monopulse method for angle measurement. This process involves forming monopulse ratios and mapping those ratios to a target angle estimate using a pre-calculated monopulse curve or polynomial. The monopulse process has not been employed for rotating radars using coherent integration because the monopulse ratios do not map to the correct target angles when curves developed for a stationary array are used.
The performance of an angle estimation technique can be gauged by its beam-splitting ratio (BSR). The BSR is defined as the antenna pattern's two-way 3-dB beamwidth divided by the standard deviation of the angle error at 20 dB SNR. Previous rotating radars employing coherent integration, such as the U.S. Government's AN/SPS-49 Long-Range Air Surveillance Radar, have measured target azimuth angle using an algorithm to locate the centroid of the detected signal envelope. However, centroiding algorithms require multiple detections using mini-dwells and are characterized by small BSRs on the order of 2 to 4. The technique described herein is applicable for radars that want to coherently integrate the whole dwell for optimum performance and achieves BSRs on the order of 8 to 10 which is over twice the typical BSR of a centroiding
U.S. Pat. No. 5,017,927, issued May 21, 1991 to Ashok K. Agrawal et al., and assigned to General Electric Co. of Morristown, N.J., discloses a technique for using phase shifters and variable gain amplifiers within the transmit-receive (TR) processor of each antenna element to compensate for errors in the internal circuitry of the sum, azimuth difference and elevation difference beam formers. This invention is designed for a non-rotating radar and is an improved hardware implementation of the original monopulse method. However, it does not correct for the effects of rotation on a radar employing coherent integration (CI).
U.S. Pat. No. 5,986,605, issued Nov. 16, 1999 to Leslie A. Priebe et al., and assigned to Raytheon Company of Lexington, Mass., discloses a new method of monopulse processing that only requires two receiver channels and does not form the traditional monopulse ratios. The antenna is still subdivided into four quadrants. Quadrant pairs are formed from the top two quadrants, the bottom two quadrants, the left quadrants and the right quadrants. The signals received on the quadrant pairs are multiplied together to form two correlation beams. The estimated elevation and azimuth angles are the phase angles of the correlation beams. Target detection is performed by thresholding the magnitude of either correlation beam. This patent disclosure is an entirely new method of monopulse processing that was designed for a non-rotating radar, and does not correct for the effects of rotation on a radar employing CI.
U.S. Pat. No. 6,618,008, issued Sep. 9, 2003, to John Arthur Scholz and assigned to Nederlandse Organisatie of Delft, Netherlands, discloses a variation on the traditional monopulse antenna architecture. The antenna is still subdivided into four quadrants and the signals received on these quadrants are still summed, differenced and divided to form monopulse ratios. However, the antenna quadrants in this invention are not fixed in place. Instead, the quadrants rotate so that the difference pattern nulls are either aligned or perpendicular to the returns from the target tracks. The inventors claim these “virtual” quadrants reduce the complication and expense of the RF hardware required and allows the target to be tracked along any angle instead of the traditional azimuth and elevation angles. This invention is designed for a non-rotating radar and is an architectural variation on the original monopulse method. However, it does not correct for the effects of rotation on a radar employing CI.
U.S. Pat. No. 6,680,687 issued Jan. 20, 2004 to Michel Phelipot and assigned to Thales of Paris, France discloses a variation on the traditional centroiding algorithm used by 2-dimensional (2D) rotating radars for estimating target azimuth. A transmitted N-pulse burst is split into two N/2-pulse half-bursts. These half-bursts are then processed to associate a signal amplitude and azimuth angle with each half-burst. Coherent integration is used to determine amplitude and centroiding is used to determine azimuth. The two half-burst measurements of amplitude and azimuth are then combined using a mathematical formula to generate an improved estimate of target azimuth. However, coherently integrating half-bursts result in a factor N/2 improvement in SNR. Coherently integrating the entire N pulse burst, as the present invention does, results in a factor N improvement in SNR. Thus, the present invention achieves 3dB more SNR. Furthermore, U.S. Pat. No. 6,680,687 is intended for a 2D radar. A 2D radar measures only range and azimuth as opposed to a 3D radar which measures range, azimuth and elevation. The present invention will work for either a 2D or 3D radar and takes into account any cross-coupling between the azimuth and elevation measurements.